Electric signals used in a high-speed optical transmitting and receiving apparatus are high-speed signals of 40 Gb/s or the like. The high-speed optical transmitting and receiving apparatus is comprised of a plurality of units. The connection between the units is connected by coaxial connectors. An example of the connection made by a coaxial connector is given below.
The unit is a printed circuit board. The printed circuit board including an earth pattern on a bottom face of the printed circuit board. The printed circuit board and an outside conductor (earth) of the coaxial connector are fixed to a metal package so that they are electrically connected (for example, refer to the Japanese Laid-open Patent Publication No. 2007-123950). The configuration of the units in the high-speed optical transmitting and receiving apparatus requires a metal package, which results in an additional cost and space for the metal package, which makes it impossible to miniaturize the configuration.
For devices used in a high-speed optical transmitting and receiving apparatus of 40 Gb/s or the like, developments for making surface-mounted type coaxial connectors which are directly connected with a transmission line (high-speed signal line) on a printed circuit board have advanced. By means of the configuration between the inside conductor and the outside conductor these coaxial connectors make the characteristic impedance constant, hence preventing deterioration of high-frequency characteristic (for example, refer to the Alan M. Lyons et al, Connector Interconnections to Transmission Lines For 40 Gb/s Broadband Applications, 2002 Electronic Components and Technology Conference, pp. 1021-1026, 2002). With the development of the surface-mounted type coaxial connectors, connection between surface-mounted connectors and printed circuit boards is being reviewed.
A printed circuit board in a high-speed optical transmitting and receiving apparatus of 40 Gb/s or the like is multifunctional. Such a multi-functioned printed circuit board is provided with a multilayer wiring substrate having therein a plurality of wiring layers. The multilayer wiring substrate includes a signal line on the top surface of the printed circuit board and a plurality of wiring layers inside the printed circuit board. The thickness of the wiring in the printed circuit board is generally about several μm to ten and several μm. To keep the characteristic impedance constant and to prevent the deterioration of a high-frequency characteristic, the signal line on the multilayer wiring substrate is generally configured with an earth pattern as a microstrip line corresponding to a high-frequency characteristic. When a microstrip line is configured with a multilayer wiring substrate, an earth pattern is arranged in the substrate in the relation with the signal line and the earth pattern.
At the connection portion of a transmission line and a coaxial connector, by using a via hole connected with an earth pattern on the bottom side surface (inside, in this case) of a microstrip line, it is generally the case that the earth pattern is pulled out onto the printed circuit board surface by the via hole in such a manner as to form a grounded coplanar configuration. At the grounded coplanar portion, a signal line formed on a printed circuit board top surface, and the earth pattern on the top surface are connected with solder with an inside conductor and an outside conductor of the coaxial connector, respectively.
As described in the Alan M. Lyons et al, to prevent deterioration of a high-frequency characteristic of 30 GHz or more, it is furthermore necessary to make a connection with the outside conductor of the coaxial connector not through the via hole of the earth pattern on the bottom face (inside, in this case) of the grounded coplanar waveguide.
However, to make direct connection of the inside earth pattern of the multilayer earth pattern with the outside conductor of the coaxial connector, it is not possible to make a connection with the outside conductor of the coaxial connector by using solder even if the inside earth pattern is exposed from the lateral face of a multilayer wiring circuit board, because the inside earth pattern is no more than ten and several micro meter (μm) thick. Furthermore, the inside earth pattern exposed from the end of the printed circuit board is located internally between the coaxial connector and the end of the multilayer wiring circuit board. Therefore, it is impossible to connect the outside conductor of the coaxial connector by using solder.
Furthermore, to expose the inside earth pattern of the multilayer wiring circuit board on the lateral face thereof, it is necessary to cut out a portion of the multilayer wiring circuit board at a position where the inside earth pattern is to be exposed. However, when a portion of the multilayer wiring circuit board is cut out, wiring in the multilayer wiring circuit board may become plasticity (the meaning of the plasticity including a tactile or malleable) deformed due to the cutting on its cut top surface. Such plasticity deformed wire may be in touch with another wiring layer. It is therefore problematic to cut the multilayer wiring circuit board.